netdev - Re: [RFC 4/7] mfd: ocelot-spi: Change the regmap stride to reflect the real one

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Date:   Fri, 24 Mar 2023 13:48:17 +0100
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rafael@...nel.org, Colin Foster <colin.foster@...advantage.com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Lee Jones <lee@...nel.org>, davem@...emloft.net,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        thomas.petazzoni@...tlin.com
Subject: Re: [RFC 4/7] mfd: ocelot-spi: Change the regmap stride to reflect
 the real one

Hello Andrew,

On Fri, 24 Mar 2023 13:11:07 +0100
Andrew Lunn <andrew@...n.ch> wrote:

> On Fri, Mar 24, 2023 at 10:36:41AM +0100, Maxime Chevallier wrote:
> > When used over SPI, the register addresses needs to be translated,
> > compared to when used over MMIO. The translation consists in
> > applying an offset with reg_base, then downshifting the registers
> > by 2. This actually changes the register stride from 4 to 1.
> > 
> > Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
> > ---
> >  drivers/mfd/ocelot-spi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/mfd/ocelot-spi.c b/drivers/mfd/ocelot-spi.c
> > index 2d1349a10ca9..107cda0544aa 100644
> > --- a/drivers/mfd/ocelot-spi.c
> > +++ b/drivers/mfd/ocelot-spi.c
> > @@ -124,7 +124,7 @@ static int ocelot_spi_initialize(struct device
> > *dev) 
> >  static const struct regmap_config ocelot_spi_regmap_config = {
> >  	.reg_bits = 24,
> > -	.reg_stride = 4,
> > +	.reg_stride = 1,
> >  	.reg_shift = REGMAP_DOWNSHIFT(2),
> >  	.val_bits = 32,  
> 
> This does not look like a bisectable change? Or did it never work
> before?

Actually this works in all cases because of "regmap: check for alignment
on translated register addresses" in this series. Before this series,
I think using a stride of 1 would have worked too, as any 4-byte-aligned
accesses are also 1-byte aligned.

But that's also why I need review on this, my understanding is that
reg_stride is used just as a check for alignment, and I couldn't test
this ocelot-related patch on the real HW, so please take it with a
grain of salt :(

Thanks,

Maxime

> 	Andrew